1. Field of the Invention
This invention relates to illumination systems, and more particularly, to an edge-lit illumination system including a hollow waveguide and one or two stackingly arranged transparent light directing arrays (LDA) that provide a light output from the illumination system having a predetermined angular distribution.
2. Description of the Prior Art
Luminaires or illumination systems employed in an office environment must be suited for both conventional paper tasks, which are typically performed on a horizontal surface with the viewer's eyes directed downward, and for computer-based or visual display terminal (VDT) tasks, which are typically performed with the viewer's eyes directed toward the VDT and which include part of the ceiling in the field of view. For VDT tasks, it is therefore important to control the luminance of luminaires installed in the ceiling to minimize reflected glare or veiling reflections on a VDT. As a general rule of thumb, the ceiling luminance should not exceed ten (10) times that of the VDT screen. See, e.g. American National Standard Practice For Office Lighting, ANSI/IESNA RP-1-1993, pp. 34-41. Ceiling luminaires may cause: image glare, where the viewer sees the image of the illumination system on the VDT; area glare, where an area of brightness appears on the VDT; and uniform glare, which results in a reduction of the brightness and contrast of the VDT. Each of these glare problems adversely effects a persons ability to perform VDT tasks. For indirect luminaires, a generally widespread light distribution and uniform light output brightness will address glare problems. In addition, undesirable glare effects will be further reduced by controlling the light output from the luminaire at viewing angles ranging from between 55.degree. and 90.degree. (e.g. it is recommended that the average luminance in this angular viewing range not exceed 850 cd/m.sup.2 when viewed in the lengthwise and crosswise directions and at an angle of 45.degree. to the lengthwise and crosswise directions). Consequently, it is desirable to provide an illumination system that can provide a controllable and relatively precise or "clean" cut-off angle; the cut-off angle being that angle beyond which light output from the illumination system is essentially insignificant or visually unnoticeable.
Glare problems from ceiling luminaires are exacerbated by the continued development of brighter, more compact and more efficient light sources. Some of these newly developed light sources may cause damage to a person's eyes, if viewed directly. In addition, bright light directly entering a person's eyes will lead to fatigue and lower worker productivity. For these reasons, it is highly desirable to provide an illumination system having a controllable and clean cut-off angle.
Illumination systems provide a means for redirecting light output from a light source to a specific location. For light sources having a narrow angular output distribution, U.S. Pat. Nos. 4,984,114 to Cobb et al. (the '144 patent) and 5,190,370 to Miller et al. (the '370 patent) disclose hollow illumination systems that include a plurality of prisms within which light rays from the light source are totally internally reflected before emerging from the illumination system. The light source emits either a cone (the '144 patent) or a collimated beam of light (the '370 patent) which contacts the prisms at a small angle with respect to the plane of the output surface of the illumination system. Virtually all light rays emitted by the light source enter the prisms and emerge from the illumination system. The angular distribution of the light output is controlled by the limited angular output of the light source and the prisms merely provide the means by which the light rays emerge from the illumination system. Consequently, the illumination systems disclosed in these references are suited only for light sources that emit narrow beams or columns of light.
For light sources having a broad or disperse angular distribution of light rays, U.S. Pat. No. 5,676,453 to Parkyn, Jr. et al. (the '453 patent) discloses an illumination system including a collimating totally-internally-reflective lens that redirects light rays from a fluorescent light source to a light target zone. While the lens redirects, and in some way, orients the light rays from the light source as they emerge from the illumination system, the uniformity of the output is not controlled by the lens disclosed in this reference and consequently, light sources may appear as "hot spots". Even though this references discloses tailoring or controlling one direction of angular distribution of light output, clean cut-off angles of greater than approximately 60.degree. are not possible.
A luminare having two generally orthogonally arranged corrugated sheets is disclosed in U.S. Pat. No. 4,452.449 (the '449 patent) to Whitehead. The corrugated sheets disclosed in this reference are characterized by a property referred to as octature--wherein surfaces on the same side of a corrugated sheet are either parallel or perpendicular to one another, and surfaces on opposite sides of the corrugated sheet are at 45.degree. to each other. Consequently, the included angle of the prisms of the corrugated sheets is necessarily limited to 90.degree.. This reference further discloses the desirability of concentrating the light output from the luminaire over the angular viewing range between 0.degree. and 30.degree. from normal.
There exists a need in the art for an illumination system that accepts light input from a variety of different light sources and that provides a light output having a predetermined angular distribution with a clean cut-off angle and good light output uniformity.